To outrenoir - Symphonic prog is a deliberate attemp to replicate the use of symphonic arrangements but using rock instruments.Not all 'Yes' songs are Symphonic prog though, who have played a very diverse range of music during their time.Most of the time with at least, an element of progressiveness in their songs.Not always though.Symphonic rock isn't necesarily progressive either.Symphonic prog uses it as it's primary musical template though.Symphonic music is generally progressive in nature as is the more classical / symphonic end of jazz.Not all jazz is progressive, again, though.Hope that gets you thinking a bit.It's not that easy to explain, unless you know the music..Alot of modern prog (since the mid 90's) isn't that symphonic at all - it's still progressive.Alot of '70's prog wasn't symphonic for that matter even.

if the characteristics of this are identical to that of regular prog why bother making this a subgenre? Surely anything else (Neo-prog, avant-prog and whatnot) are the only ones that need distinguishing from common prog rock with different terms, and those aren't particularly "rock" anyway, like this isn't particularly "symphonic".